1. Field of the Invention
This invention relates to a ball screw apparatus of a screw shaft circulating-type in which a circulating groove is formed in a screw shaft, and interconnects opposite ends in a spiral direction of a thread groove of the screw shaft to form a closed-loop ball endless circulating passageway.
2. Description of the Background Art
For example, a member which is called “a circulating piece”, having a ball circulating groove, has heretofore been used in a ball circulating system for achieving a compact design of a ball screw apparatus by decreasing an outer diameter of a nut.
The circulating piece is fitted in a fitting hole formed in an outer peripheral surface of the nut, and is fixed thereto to form an endless circulating passageway for balls. By a scooping-up portion of the circulating piece, the balls which roll in a load track defined between a thread groove of the nut and a thread groove of a screw shaft, are moved over a land portion of the threaded groove, and are guided into the circulating groove within the circulating piece, and are returned to the adjacent load track between the two thread grooves via the circulating groove.
However, in the ball circulating system employing such a circulating piece, it was necessary to use the externally-mounted part which corresponds to the circulating piece, and besides it was necessary to form the fitting hole for mounting the circulating piece in the nut. Furthermore, a positioning operation for properly positioning connecting portions of the circulating groove and the thread groove with each other when mounting the circulating piece in the fitting hole of the nut was very cumbersome, so that the cost becomes increased.
Under the circumstances, there has been proposed a ball screw apparatus of a screw shaft circulating-type in which a circulating groove is formed in a screw shaft, and interconnects opposite ends in a spiral direction of a thread groove of the screw shaft to form a closed-loop ball endless circulating passageway, thereby achieving the endless circulation of balls with an inexpensive and simple construction without using an externally-mounted part such as a circulating piece (see, for example, Japanese Patent Unexamined Publication JP-A-2003-74665).
However, the following problem is encountered in the case where the circulating groove is directly formed in the screw shaft without using the circulating piece as in the ball screw apparatus of the screw shaft circulating-type disclosed in the JP-A-2003-74665. Namely, a tool which is driven for rotation so as to process the thread groove, such as a cutting tool and a grinding stone, is needed to be reduced in diameter size so that the tool will not interfere with the circulating groove. More specifically, when the thread groove is processed in a direction of arrow D (FIG. 7), using the rotary tool with a large diameter, there is a fear that the rotary tool interferes with a portion C of the screw shaft, and the rotary tool processes or machines this portion C. In contrast, when the feeding of the rotary tool in the direction of arrow D is stopped at a region short of the circulating groove so that the portion C will not be processed by the rotary tool, a portion B of the screw shaft is not properly processed. Therefore, it was necessary to reduce the diameter of the rotary tool so that the portion C would not be processed, while the portion B could be positively processed. Therefore, the efficiency of processing the screw shaft was lowered, and this prevented the production cost from being reduced.